Zebrafish (Danio rerio) is a small tropical freshwater fish, and belongs to the family of Cyprinidae. They live in clean ponds in fields, and origins from the East India Ganges River valley, including Pakistan, India, Nepal and Bangladesh area. The appearance of their body has silver color as the background, and topped with blue stripes extending from head to tail; this is the main reason why they are called “zebra” fish. The average of the fish's body length is between 3 and 5 cm. Generally, the adult male body is slender and more yellowish in color; the adult female has more silver in color and the female body size is bigger with the extended abdomen containing oocysts inside. Zebrafish grow up from the fertilized egg to reproductive adulthood in about 3 months, and their average life is about 2 to 3 years. In nowadays, zebrafish is a popular aquarium fish, frequently admired by their body's shiny silver, blue and white stripes that scatters reflected bright light occasionally. The typical aquariums sell not only the wild typed strains, but also some special variant strains, such as the long tail fin zebrafish, golden zebrafish, leopard zebrafish and many other mutant varieties.
Outside the hobbyist's aquarium, zebrafish is also a popular model organism in biomedical research. Researchers used zebrafish to study developmental biology, revealing the functions of specific genes during development; zebrafish can also be used as human disease models to simulate various disease processes. The zebrafish model organism takes the biggest advantage of their “transparent” body during embryonic stage; it can provide live images for real-time observations, allowing researchers to clearly monitor the whole process of pathogenesis. However, the characteristic “transparent” appearance in zebrafish is limited only in early embryonic stage; with the individual growth, pigment cells would gradually cover the whole skin, and eventually obstruct direct observation, which is inconvenient for researchers. If technically, the transparent body can be maintained into the adult stage, it will be a major advantage to expand research capacity. Furthermore, adult animals present biological conditions closer to the true situations of a variety of disease models. After all, the zebrafish embryo represents the earliest stage of life and is a unique model, with many of their physiological mechanisms and organ functions different from the adult ones. Their small size also presents restrictions for some experimental manipulation, such as surgical operations.
At the late 1960s, Prof George Streisinger at the University of Oregon introduced zebrafish into the research laboratory and academic community. He described the developmental process in zebrafish, and defined the various developmental stages in detail. Such knowledge is useful for other researchers as references. Since then, zebrafish began to be widely used in developmental biology research. Being a favorable model organism, zebrafish possess many advantages: the maintenance costs are cheaper compared with other vertebrate model organisms such as mice or monkeys. In addition, zebrafish engage in courtship and spawn in the morning, so researchers can artificially manipulate light cycle to control the mating behavior of the zebrafish; furthermore, healthy female can lay a batch of about 200 to 300 eggs each time, which are enough to provide sufficient experimental materials and statistically significant sample size. Most important of all, the transparent zebrafish embryo is developed outside of the mother (i.e. not in the womb), which allows real time observation. Finally, zebrafish and mammals are all grouped in the chordate phylum, and thus have similar physiological features such as closed-type blood reservoir and circulation system, and congenital and acquired immune system. Zebrafish also possess many homologous organs similar to mammals, such as brain, heart, liver, pancreas, kidney, gastrointestinal tract, etc. These physiological characteristics are difficult to be completely owned in other simpler model organisms.